Additive for liquid crystal material

ABSTRACT

A nematic liquid crystal material which assumes a homeotropic texture is provided by dissolving an additive material having the formula RR3&#39;&#39;N X wherein R is an alkyl radical having 10 to 24 carbon atoms, R&#39;&#39; is a methyl or ethyl radical, and X is an anion derived from a simple acid, in a nematic material. A typical example of the additive is hexadecyltrimethylammonium bromide. The homeotropic texture provided by this composition, when placed between two conducting transparent plates, is readily deformed by a voltage applied to the plates and can thereby be utilized to control the transmission of light.

United States Patent Haller et al.

[151 3,656,834 1 Apr. 18,1972

ADDITIVE FOR LIQUID CRYSTAL MATERIAL Castellano ..350/160 PrimaryExamine rRonald L. Wibert Assistant Examiner-Edward S. BauerAttorney-Hanifin and J ancin and Isidore Match 57 ABSTRACT A nematicliquid crystal material which assumes a homeotropic texture is providedby dissolving an additive material having the formula wherein R is analkyl radical having 10 to 24 carbon atoms, R is a methyl or ethylradical, and X is an anion derived from a simple acid, in a nematicmaterial. A typical example of the additive ishexadecyltrimethylammonium bromide. The homeotropic texture provided bythis composition, when placed between two conducting transparent plates,is readily ,deformed by a voltage applied to the plates and can therebybe utilized to control the transmission of light.

10 Claims, 4 Drawing Figures PATENTEDAPRIBIQH v 3,656,834

FIG.2

ORIENTATION 0F MOLECULES 0F NEMATIC FILM 24 INVENTORS IVAN HALLER HAROLDA. HUGGINS BY 9W 7mm- ATTORNEY ADDITIVE FOR LIQUID CRYSTAL MATERIALBACKGROUND OF THE INVENTION This invention relates to liquid crystals.More particularly, it relates to novel liquid crystal compositionswherein the orientation of films thereof can be controlled. It isconcerned with nematic liquids which are a type of liquid crystals, theother two types being smectic and cholesteric.

Nematic liquids are characterized by a structure which results from theparallel orientational ordering of the longitudinal axes of the rod'likemolecules which constitute these liquids. Such ordering obtains fordistances which are substantially greater than the molecular dimensionsbut the size and arrangement of regions of uniform orientation aregreatly influenced by the boundary surfaces. Nematic liquid crystals areanisotropic in their optical, electrical and magnetic properties. Sincethe forces responsible for the orientational order are much smaller thanthe corresponding forces responsible for the structure of solids, thedetailed structure of a nematic liquid is more readily influenced byexternal agencies of such electric and magnetic fields. This propertygives rise to pronounced electro-optical effects which can beadvantageously employed in display devices.

The employment of nematic liquids as an electro-optical element formodulating light or displaying light is set forth in US. Pat. 3,322,485that issued to Richard V. Williams on May 30, 1967. A liquid crystaldisplay element is disclosed in US. Application of Marvin J. Freiser andIvan Haller for Liquid Crystal Display Element, Ser. No. 875,235 filedNov. 10, 1969. R.

In the absence of external aligning influences, the direction of theorientational order in a nematic liquid, i.e. the direction of the opticaxis varies randomly from place to place. The variation is generallyboth continuous and abrupt; the location of abrupt changes inorientation being termed disinclination lines. In the vicinity of aboundary surface, the orientation direction of the liquid crystal isdetermined by the nature and past history of the surface. For example,most nematic liquids in contact with an untreated glass surface orientin the surface layer with the long axis of the molecules lying in theplane of the surface. The orientation direction in the plane varies,however, from point to point, subdividing the surface into randomregions or domains of different orientation.

When a nematic liquid is placed between two surfaces, spaced apart about-1000 microns, it exhibits one of several kinds of characteristicbirefringence patterns, termed textures. These textures can be observedvisually at low magnification between crossed polarizers. They resultfrom the variation of the optic axis direction in the nematic layerwhich in turn is determined by the orientational domain pattern of thebounding surface as well as the deformational characteristics of theliquid itself. For example, most nematic liquid crystals, when enclosedbetween two untreated glass surfaces, exhibit a threaded texture. Thethreads in this texture have their origin in the disinclination lineswhile the remainder of the texture originates in the continuousvariation in the optic axis direction in the bulk to accommodate theoptic axis directions in the boundary layer imposed by the glass.Another example is the homogeneous, or aligned, texture, which ischaracterized by the optic axis pointing uniformly in a predetermineddirection but lying in the plane of the boundary surface. This textureis prepared by surface treatments such as rubbing prior to the enclosureof the liquid crystal.

It is often desirable for device applications and in the measuring ofcertain physical properties to provide a nematic layer in a homeotropictexture, i.e., large areas in which the optic axis is orientedperpendicularly to the boundary. I-Ieretofore, homeotropic texturelayers have been prepared by pretreating the surfaces which subsequentlyenclose the nematic layer with a strongly oxidizing acid such as nitricor chromesulfuric acid. This technique has been found to be undesirablein many situations because it cannot be used with oxidation-sensitive(transparent conductive coated) surfaces. Particular care is required toavoid the touching of a surface so treated, and homeotropic nematiclayers produced by this technique tend to be unstable, i.e., theirhomeotropic property tends to decrease in an undesirably short period.

Accordingly, it is an important object of this invention to provide anematic liquid crystal material, which as a layer between two boundingsurfaces, assumes a homeotropic texture without the need for chemicallypretreating the bounding surfaces.

It is another object to provide a nematic liquid crystal material inaccordance with the preceding object which, when subjected to an appliedelectric field acting upon the dipole component perpendicular to thelong axis of the molecules, deforms the homeotropic texture.

It is a further object to provide an improved electro-optical elementsuitable for use in light modulators, optical storage or displays andthe like employing the nematical liquid crystal material at theinventory.

SUMMARY OF THE INVENTION Generally speaking and in accordance with theinvention, there is provided a nematic liquid crystal material whichassumes a homeotropic texture comprising a nematicmaterial havingdissolved therein a material having the fonnula:

RR 'N X wherein R is selected from group consisting of alkyl radicalshaving 10 to 24 carbon atoms, R is selected from the group consisting ofmethyl and ethyl and X is selected from the group consisting of anionsderived from simple acids.

Typical examples of nematic materials may be such compounds asp-methoxybenzylidene-p-butylaniline,p-methoxybenzylidene-p-aminophenylacetate, p-azoxyanisol andbutylp-(p-ethoxyphenoxycarbonyl)-phenylcarbonate. A suitable example ofR may be a hexadecyl radical. The anion may be a halide ion such as Cl,Br, I, or an anion derived from oxyacids such as No; and the like. Atypical example of the material symbolized by the structural formula maybe hexadecyltrimethyl-ammonium bromide which may be present in itssolution in the nematic material in a range of 0.25 to 2.5 percentdepending, of course, on its solubility characteristics relative to thespecific nematic material which is employed.

The foregoing and other objects, features, and advantages of theinvention will be apparent from the following more particulardescription of the preferred embodiments of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS In the drawings, FIG. 1 is a schematicdepiction of an improved nematic cell incorporating the materialaccording to the invention;

FIG. 2 is a diagram which illustrates changes in polarization of lightpassing through the cell for two states of the cell;

FIG. 3 is a modification of the cell of FIG. 1 for use with reflectedlight; and

FIG. 4 illustrates the changes in polarization of light for the twostates of the cell of FIG. 3.

DESCRIPTION OF A PREFERRED EMBODIMENT In accordance with the invention,it has been discovered that the dissolving of a small percentage of amaterial such as hexadecyltrimethyl ammonium bromide in nematicmaterials causes them to assume a homeotropic texture between glassslides, even in the absence of any treatment or cleaning of surfaces.

The additive of a specific molecular shape forms on the enclosingsurface an adsorbed monolayer of a specific molecular orientation. Thismonolayer then transmits a sense of perpendicular orientation toadjacent layers of liquid crystalline molecules. In this connection, itis believed that the polar molecules of the additive are preferentiallyadsorbed on the polar surface. They are so adsorbed in a manner thatmaximizes the energy of interaction that is in an orientation thatpermits the nitrogen atom which carries the positive charge to reside ata minimum distance from the surface. By virtue of the known tetrahedralarrangement of bonds around the quaternary nitrogen atom, this requiresthe three small R radicals to be in contact with the surface and thelarger R radical to reside on the side of the nitrogen atom opposite tothe surface and to point, in an elongated conformation, perpendicular tothe surface. Molecules of the liquid crystal that predominate in thesecond and subsequent layer, are kept, by molecular forces, parallel tothe long alkyl chains of the adsorbed layer and, consequently, areperpendicular to the boundary surface.

It has been found that a nematic material such asp-methoxybenzylidene-p-butylaniline, a room temperature nematicmaterial, containing 0.49 percent by weight of the above-mentionedhexadecyltrimethyl ammonium bromide additive exhibits an almost perfecthomeotropic texture. Homeotropy has also been produced with the latterammonium compound where the nematic material which is employed isp-methox ybenzylidene-p aminophenylacetate containing 0.96 percent ofthe additive. The nematic material p-azoxyanisol containing 0.48 percentof the additive showed a homeotropic texture in the temperature range ofapproximately 2C below the nematic isotropic transition temperature. Thenematic material butyl-p-(p-ethoxyphenoxy carbonyl) phenylcarbonatehaving the additive dissolved therein also showed homeotropy.

Among the advantages presented by the achieving of homeotropy with anadditive as opposed to the method of treating the bonding surfaces withoxidizing acids are the following:

l. Oxidation-sensitive transparent conductive surfaces can be used.

2. No special care is required to avoid the touching of the treatedsurface.

3. The stability of the treated surface relative to time of thehomeotropic surface shows superior qualities.

A display element utilizing a homeotropic nematic liquid crystalmaterial made according to the invention, can be provided by enclosingthe latter material between tin-oxide coated, i.e., conductive, platesof a transparent material such as glass or plastic and viewing thestructure between crossed polarizers in transmitted light. In theabsence of an electric field, the device appears dark as the optic axisof the material lies in the propagation direction of the light. Anapplied electric field acting upon the dipole component perpendicular tothe long axis of the molecule deforms the homeotropic texture. As theoptic axis no longer lies in the propagation direction, the light isdepolarized and the element appears bright.

There follows hereinbelow a detailed description of the making andtesting of the inventive homeotropic nematic material.

EXAMPLE I An 0.49 percent solution of hexadecyltrimethyl ammoniumbromide in p-methoxybenzylidene-p-butylaniline was prepared by mixing0.0024 and 0.495 grams, respectively, of the materials in a vial,heating to a temperature where the nematic liquid first turned isotropic(approximately 46C) and shaking until the last specks of the solid massdissolved. A drop of the solution at room temperature was placed betweenuntreated glass plates whose separation was effected by a 2- mil thickteflon gasket. The sample was then examined between crossed polars on apolarizing microscope over its entire nematic temperature range andextinction was observed independently of the direction of the samplerelative to the polarization direction. Upon the shearing of the liquidcrystal by selective motion of the two plates, light was observed to betransmitted. These observations indicated that the optic axis of thenematic fluid, at rest, is perpendicular to the glass plates, i.e.,because of the additive, it assumed a homeotropic texture.

In a second experiment, the same procedure as described above wasfollowed with the exception that the glass slides which were employedwere first repeatedly rubbed on a cheese cloth in a given direction.Such rubbing treatment is known to normally cause purep-methoxybenzylidene-p-butylaniline, which is subsequently enclosed, toassume a texture in which the optic axis lies in the plane of the glassand in the rubbing direction. The inventive solution, as describedhereinabove, however, was again observed to assume the hometropictexture.

In a third experiment, the same solution was enclosed betweenelectrically conducting tin oxide-coated glass plates wherein it againexhibited a homeotropic texture. A dc voltage was then applied to theplates, while the appearance of the sample was monitored on thepolarizing microscope and the intensity of the transmitted light(between crossed polarizers) was measured with a CdS photocell replacingone of the eyepieces of the binocular microscope. With an appliedvoltage of 0 to 5 volts, the structure appeared uniformly dark. At 7.5volts however, the liquid crystal material became transparent and, undermagnification, it appeared stationary.

The contrast ratio, i.e., transmitted light intensity at 7.5 voltsrelative to that at zero volts was found to be 16. With applied voltagesof 10 volts or higher, flow patterns and turbulence appeared similar tothose patterns and turbulence observed in nematic materials of anytexture. The maximum contrast which was achieved was 31 at an appliedvoltage of 24 volts. In a comparison, the maximum contrast ratiomeasured under similar conditions, of elements containing nematic liquidcrystals with no additive, and aligned in the plane of the boundarysurfaces in the polarization direction was found to be only 11 to 17.

EXAMPLE 2 Hexadecyltrimethylammonium bromide was dissolved inbutyl-p-(p-ethoxyphenoxy carbonylphenyl carbonate in a concentration of0.4 to 1.1 percent by weight, to provide the inventive nematic liquidcrystal material.

EXAMPLE 3 Hexadecyltrimethylarnmonium bromide was dissolved inpmethoxybenzylidene-p-aminophenyl acetate in a concentration of 0.4 to1.1 percent by weight to provide the inventive nematic liquid crystalmaterial. A sample containing 0.95 percent of the additive showedparticularly good results when tested.

EXAMPLE 4 Hexadecyltrimethylammonium bromide was dissolved inpazoxyanisol in a concentration of 0.48 percent by weight to provide theinventive nematic liquid crystal material.

The testing of the samples produced in Examples 2, 3, and 4 followed thesame procedures as set forth in Example I, and similar good results wereobtained.

Referring now to the drawings wherein there is illustrated an improvednematic cell utilizing the nematic material prepared according to theinvention, the electro-optical device 2 of FIG. 1 comprises twotransparent substrates 4 and 6, such as glass or other inert materialhaving good light transmissive characteristics. The substrates 4 and 6have plane opposed parallel faces on which there are depositedelectrically conductive light-transparent electrodes 8 and 10, of theorder of 1000 3000A in thickness. There are many materials that can beemployed for making such electrodes; tin oxide being one of thesesuitable materials.

The spacing between electroded glass plates 4 and 6 is between 10 1000microns, this spacing being maintained by shims, raised bevels on theopposed parallel plates 4 and 6, or by any other suitable means.Although capillary action will be sufiicient to maintain liquid crystal12 in the thin space between electrodes 8 and 10, after electrical leadsl4 and 16 have been connected to electrodes 8 and 10, respectively,

molten glass heads 18 and 20 can be dropped over the spacings to preventloss of fluid. Secured to the outer surfaces of each glass plate 6 and 4are crossed polarizers 22 and 24. A dc or relatively low frequency (upto several kilocycles) potential source is connected to leads 14 and 16.

Reference is now made to FIG. 2 which is of assistance in theexplanation of the operation of the arrangement shown in FIG. 1. In thisoperation, let it be assumed that a diffused light source 28 is observedthrough cell 2 at a location in front of cell 2 as shown. Such diffusedlight 28, when passing through polarizer 22, will become polarized in adirection parallel to the plane of the drawings. Upon passing throughthe nematic liquid 12 in its quiescent state (switch 30 is open and novoltage is across electrodes 8 and the light remains polarized in theplane of the paper. However, since polarizer or analyzer 24 is crossedwith polarizer 22, such polarized light does not pass through analyzer24 and the observer sees a dark background despite the presence ofambient light in the neighborhood of cell 2.

The embodiment of FIG. 3 is similar to the cell of FIG. 1 with thedifference that the polarized light that traverses cell 2, impinges on amirror 32, and then returns through the cell back to the eye of theobserver. As seen in FIG. '4, the diffused light source 28 enters cell 2from the same side of the observer. Such difiused light, on passingthrough analyzer 24, becomes polarized perpendicular to the plane of thedrawing. With switch 30 closed, the polarized light traverses cell 2unchanged, but is unable to pass through polarizer 22 that is crossedwith polarizer 24. Consequently, no light is reflected from mirror 32,whereby the observer sees cell 2 as an opaque background. When switch 30is closed so as to activate cell 2, as seen in the bottom portion ofFIG. 4, diffused light source 28 becomes polarized perpendicularly tothe plane of the drawing as it passes through polarizer 24, but isdepolarizedafter' passing through the now activated cell 2. Suchdepolarized light, upon passing through polarizer 22, becomes polarizedin the plant of the drawing and remains so polarized after reflectionfrom mirror 32 and passage again through polarizer 22. The polarizedlight is again depolarized upon its passage through active cell 2 andbecomes depolarized again, exiting from analyzer 24 as light that ispolarized at right angles to the plane of the paper and observable bythe viewer.

The horizontal lines in nematic material 12 are intended toschematically indicate that the nematic material being used in thedevices of FIGS. 1 and 3 is the material prepared according to theinvention as described hereinabove. The material 12 has a thresholdelectric field which, when exceeded, causes the deformation of thehomeotropic texture and thereby the depolarization of polarized lightpassing therethrough. When an electric field is applied across thenematic crystal material which exceeds that threshold, depolarization ofpolarized light passing therethrough takes place.

While the invention has been particularly described with reference topreferred embodiments thereof, it will be understood by those skilled inthe art that the foregoing and other changes in form and details may bemade therein without departing from the spirit and scope of theinvention.

What is claimed is:

1. A nematic liquid crystal material which assumes a homeotropic texturecomprising:

a nematic material having dissolved therein an additive having theformula RR 'N*X wherein R is selected from the group consisting of alkylradicals having 10 to 24 carbon atoms, R is selected from the groupconsisting of ethyl and methyl, and X- is selected from the groupconsisting of anions derived from simple acids.

2. A nematic liquid crystal. material which assumes a homeotropictexture comprising:

a nematic material having dissolved therein a material having theformula R(CH N X wherein R is selected fromthe group consisting of alkylradicals having 10 and 24 carbon atoms and X is selected from the groupconsistingof anions derived from simple acids.

3. A nematic liquid crystal material which assumes a homeotropic texturecomprising:

a nematic material having dissolved therein hexadecyltrimethylammoniumbromide.

4. A nematic liquid crystal material as defined in claim 3 wherein saidnematic material is selected from the group consisting ofp-methoxybenzylidene-p-butylaniline,p-methoxybenzylidene-p-aminophenylacetate, pazoxyanisol andp-(pethoxyphenoxycarbonyl)-phenyl carbonate.

5. A nematic liquid crystal material as defined in claim 4 wherein saidhexadecyltrimethylammonium bromide is about 0.25 to 2.5 percent byweight of its solution in said nematic material.

6. A nematic liquid crystal material which assumes a homeotropic texturecomprising about an 0.49 percent solution of hexadecyltrimethylammoniumbromide in p-methoxybenzylidine-p-butylaniline.

7. An electro-optical display device comprising:

two parallel transparent members spaced apart from 10 1000 microns;

transparent electrodes on the facing surfaces of said members;

a nematic liquid crystal material interposed and filling the spacebetween said electrodes and having a homeotropic texture, said nematicliquid crystal material comprising a nematic material having dissolvedtherein an additive having the formula RR 'N X wherein R is selectedfrom the group consisting of alkyl radicals having 10 to 24 carbonatoms, R' is selected from the group consisting of ethyl and methyl, andX is selected from the group consisting of anions derived from simpleacids;

two crossed polarizers located adjacent said transparent members, eachpolarizer located on the outer surface of its respective transparentmember; and

an electrical potential source connected to said electrodes and adaptedto apply a voltage to said electrodes.

8. An electro-optical display device comprising:

two parallel transparent members spaced apart about 10 1000 microns;

transparent electrodes on the facing surfaces of said members;

a nematic liquid crystal material interposed and filling the spacebetween said electrodes and having a homeotropic texture, said nematicliquid crystal material having a threshold electric field which, whenexceeded, causes deformation of said homeotropic texture and, thereby,depolarization of polarized light passing through, said nematic liquidcrystal material comprising a nematic material having dissolved thereinan additive having the formula RR 'N wherein R is selected from thegroup consisting of alkyl radicals having 10 to 24 carbon atoms, R isselected from the group consisting of ethyl and methyl, and X isselected from the group consisting of anions derived from simple acids;

two crossed polarizers located on the outer surface of its respectivetransparent member; and

means for applying an electric field across nematic liquid crystalmaterial through said electrodes that exceeds said threshold electricfield.

9. An electro-optical display device comprising:

two parallel transparent members spaced apart between 10 1000 microns;

transparent electrodes on the facing surfaces of said mem bers;

a nematic liquid crystal material interposed and filling the spacebetween said electrodes and having a homeotropic nematic material havingdissolved therein an additive having the formula RR 'N X wherein R isselected from the group consisting of alkyl radicals having 10 to 24carbon atoms, R is selected from the group consisting of ethyl andmethyl, and X is selected from the group consisting of anions derivedfrom simple acids;

texture, said nematic liquid crystal material comprising a texture, saidnematic liquid crystal material having a threshold electric field which,when exceeded, causes deformation of said texture and therebydepolarization of polarized light passing therethrough;

said nematic liquid crystal material comprising a nematic materialhaving dissolved therein an additive having the formula RR 'N X whereinR is selected from the group consisting of alkyl radicals having 10 to24 carbon atoms, R is selected from the group consisting of ethyl andmethyl, and X is selected from the group consisting of anions derivedfrom simple acids;

two crossed polarizers located adjacent said transparent members, eachpolarizer located on the outer surface of its respective transparentmember;

a mirror adjacent one of said polarizers; and

means for applying an electric field across said nematic liquid crystalmaterial through said electrodes that exceeds said threshold electricfield.

2. A nematic liquid crystal material which assumes a homeotropic texturecomprising: a nematic material having dissolved therein a materialhaving the formula R(CH3)3N X wherein R is selected from the groupconsisting of alkyl radicals having 10 and 24 carbon atoms and X isselected from the group consisting of anions derived from simple acids.3. A nematic liquid crystal material which assumes a homeotropic texturecomprising: a nematic material having dissolved thereinhexadecyltrimethylammonium bromide.
 4. A nematic liquid crystal materialas defined in claim 3 wherein said nematic material is selected from thegroup consisting of p-methoxybenzylidene-p-butylaniline,p-methoxybenzylidene-p-aminophenylacetate, p-azoxyanisol andp-(p-ethoxyphenoxycarbonyl)-phenyl carbonate.
 5. A nematic liquidcrystal material as defined in claim 4 wherein saidhexadecyltrimethylammonium bromide is about 0.25 to 2.5 percent byweight of its solution in said nematic material.
 6. A nematic liquidcrystal material which assumes a homeotropic texture comprising about an0.49 percent solution of hexadecyltrimethylammonium bromide inp-methoxybenzylidine-p-butylaniline.
 7. An electro-optical displaydevice comprising: two parallel transparent members spaced apart from10 - 1000 microns; transparent electrodes on the facing surfaces of saidmembers; a nematic liquid crystal material interposed and filling thespace between said electrodes and having a homeotropic texture, saidnematic liquid crystal material comprising a nematic material havingdissolved therein an additive having the formula RR3''N X wherein R isselected from the group consisting of alkyl radicals having 10 to 24carbon atoms, R'' is selected from the group consisting of ethyl andmethyl, and X is selected from the group consisting of anions derivedfrom simple acids; two crossed polarizers located adjacent saidtransparent members, each polarizer located on the outer surface of itsrespective transparent member; and an electrical potential sourceconnected to said electrodes and adapted to apply a voltage to saidelectrodes.
 8. An electro-optical display device comprising: twoparallel transparent members spaced apart about 10 - 1000 microns;transparent electrodes on the facing surfaces of said members; a nematicliquid crystal material interposed and filling the space between saidelectrodes and having a homeotropic texture, said nematic liquid crystalmaterial having a threshold electric field which, when exceeded, causesdeformation of said homeotropic texture and, thereby, depolarization ofpolarized light passing through, said nematic liquid crystal materialcomprising a nematic material having dissolved therEin an additivehaving the formula RR3''N X wherein R is selected from the groupconsisting of alkyl radicals having 10 to 24 carbon atoms, R'' isselected from the group consisting of ethyl and methyl, and X isselected from the group consisting of anions derived from simple acids;two crossed polarizers located on the outer surface of its respectivetransparent member; and means for applying an electric field acrossnematic liquid crystal material through said electrodes that exceedssaid threshold electric field.
 9. An electro-optical display devicecomprising: two parallel transparent members spaced apart between 10 -1000 microns; transparent electrodes on the facing surfaces of saidmembers; a nematic liquid crystal material interposed and filling thespace between said electrodes and having a homeotropic texture, saidnematic liquid crystal material comprising a nematic material havingdissolved therein an additive having the formula RR3''N X wherein R isselected from the group consisting of alkyl radicals having 10 to 24carbon atoms, R'' is selected from the group consisting of ethyl andmethyl, and X is selected from the group consisting of anions derivedfrom simple acids; two crossed polarizers located adjacent saidtransparent members, each polarizer located on the outer surface of itsrespective transparent member; a mirror adjacent one of said polarizers;and an electrical potential source connected to said electrodes andadapted to apply a voltage to said electrodes.
 10. An electro-opticaldisplay device comprising: two parallel transparent members spaced apartbetween 10 - 1000 microns; transparent electrodes on the facing surfacesof said members; a nematic liquid crystal material interposed andfilling the spaces between said electrodes and having a homeotropictexture, said nematic liquid crystal material having a thresholdelectric field which, when exceeded, causes deformation of said textureand thereby depolarization of polarized light passing therethrough; saidnematic liquid crystal material comprising a nematic material havingdissolved therein an additive having the formula RR3''N X wherein R isselected from the group consisting of alkyl radicals having 10 to 24carbon atoms, R'' is selected from the group consisting of ethyl andmethyl, and X is selected from the group consisting of anions derivedfrom simple acids; two crossed polarizers located adjacent saidtransparent members, each polarizer located on the outer surface of itsrespective transparent member; a mirror adjacent one of said polarizers;and means for applying an electric field across said nematic liquidcrystal material through said electrodes that exceeds said thresholdelectric field.